Visualization of tomographic data using surface rendered "3D" transparency displays is of potential use in surgical planing and in clinical evaluation of organ function. We are investigating the use of 3D visualization techniques (using the AVS package) for assistance in surgical planning (removal of lesions in the kidneys) in von Hippel Lindau disease, using CT data. The CT slices are redisplayed as a 3D volume which can be viewed from any angle. Lesions within the kidney are identified with a "seed" point, as is the collecting system. Region growing techniques and various morphological operators are used to demarcate the tumor and the collection system. It is desired to develop tools to allow the surgeon to determine the optimal surgical path for lesion removal, and to allow the radiologist to determine lesion volume. Similar 3D techniques are being investigated for display of SPECT gated cardiac bloodpool data, permitting display of 3D surface maps of the beating heart. It is hoped that using the 3D display technique in this manner may increase the clinical utility of gated blood pool scintigraphy for the assessment of cardiac ventricular function.